A solder-bearing lead is known in which the lead includes opposed resilient clamping fingers at one end of an elongated stem, with at least one of the fingers defining an electrical contact. The clamping fingers include opposed inner surfaces which define a gap for the reception of a rigid substrate circuit device therebetween, such that the inner surface on the electrical contact clamping finger engages a contact pad on the substrate circuit device. On an outer opposite surface of the contact clamping finger, the contact clamping finger carries a solder preform. The solder preform, upon being temporarily subjected to heat in a soldering operation, initially melts and flows over opposite sides of the contact finger onto the contact pad, and then resolidifies to bond the lead to the contact pad. Solder-bearing leads of this type are shown in the U.S. Pat. No. 4,019,803 to M. S. Schell, and the U.S. Pat. Nos. 4,120,558 and 4,203,648 to J. Seidler.
Solder-bearing leads as above described normally are fabricated in strip form in a progressive punch-and-die from a strip of phosphor bronze base metal which has been provided with thin tin coatings on opposite sides thereof, to facilitate the subsequent making of electrical connections to the leads. During the lead fabrication process in the progressive punch-and-die, a continuous solder wire is attached to the contact fingers of the leads and subsequently clipped between the leads to form the solder preforms on the leads. Further, during the lead fabrication process the stems of the leads are formed integrally connected to an elongated continuous support rail which subsequently is clipped from the stems after the leads have been mounted on a substrate circuit device and soldered to respective contact pads on the device.
In fabricating the solder-bearing leads from a tin-coated phosphor bronze strip as above-described, while portions of one of the tin coatings are wiped across a portion of the thickness of the base metal so as to coat the base metal, a major portion of the base metal thickness usually is not coated in this manner and is exposed to the atmosphere. When each lead then is mounted on a substrate circuit device the portion of the solder preform held by the contact finger of the lead is separated from the contact pad with which the contact finger is engaged, by phosphor bronze base metal which, as a result of oxidation from exposure to the atmosphere, is not readily wetted by solder. Accordingly, where the lead stem is horizontally disposed, instead of flowing down onto the contact pad in a soldering operation, the solder from the melted preform normally flows along the solder-wettable tin coating of the lead on the lead stem in a "wicking" action. Similarly, molten solder which does flow down to the contact pad then flows along the solder-wettable tin coating on the opposite side of the lead stem in a "wicking" action. Either flow is undesirable because the solder contaminates the tin coatings on the lead stem and because sufficient solder then may not be available on the contact pad to form a satisfactory soldered connection between the contact pad and the lead.
The flow of the molten solder along a stem of the lead tends to occur even when the stem is disposed in a vertical position during the soldering operation, with the melted solder from the preform flowing up opposite sides of the stem in a "wicking" action. While the molten solder then usually flows back down the stem onto the contact pad, the reverse flow of the solder has the further disadvantage of causing detrimental melting and thinning out of the tin coatings on the stem. Further, in certain instances where the solder resolidifies on the stem, sufficient solder may not be available on the contact pad, as noted above, to form a satisfactory soldered connection between the contact pad and the lead.
Accordingly, a primary purpose of this invention is to provide a new and improved solder-bearing lead having solder-confining stops on a stem of the lead to preclude detrimental flow of molten solder from a solder preform along the stem.